1. Field of the Invention
The present invention relates to a roll for manufacturing a float glass. More particularly, it relates to a lift-out roll and conveyor roll for taking up and conveying a float glass ribbon formed on a bath of molten metal.
2. Description of the Prior Art
In the manufacture of a float glass, a molten glass is continuously fed from a glass melting furnace onto the surface of the bath of molten metal such as tin or tin alloy kept in non-oxidizing atmosphere.
The molten glass is floated on the bath of molten metal in a form of a continuous ribbon and it is forwardly transferred with a predetermined width and thickness to be smooth and flat and then, it is cooled and taken out from an outlet of a tank containing the bath of molten metal and is transferred to an annealing lehr by rolls.
Rolls for transferring the glass ribbon are disposed between the outlet of the glass in the tank of the bath of molten metal and the annealing lehr. The roll which is brought into contact with the lower surface of the glass ribbon immediately after being taken out from the bath to support and transfer the glass ribbon is called a lift-out conveyor roll or lift-out roll.
The lift-out roll is always brought into contact with the glass ribbon at high temperature such as about 550.degree. to 650.degree. C. and is also brought into contact with the molten tin adhered on the lower surface of the glass ribbon whereby the lift-out roll should have high temperature strength and excellent oxidation resistance and excellent corrosion resistance.
Heretofore, the lift-out roll is usually made of a product of JIS-SCH22 or SCH21 heat-resistant cast steel which corresponds to ASTM HK40 or HK30 known as 25 Cr-20 Ni heat-resistant cast steel.
The 25 Cr-20 Ni heat-resistant cast steel has a composition consisting essentially of 0.2-0.45 wt.% of C; up to 2.0 wt.% of Si; up to 2 wt.% of Mn; up to 0.04 wt.% of P; up to 0.04 wt.% of S; 19 to 22 wt.% of Ni; 23 to 27 wt.% of Cr and a balance of Fe and impurities.
It has been also proposed to use an improved steel containing less than 0.5 wt.% of Mo.
The high temperature mechanical strength of the lift-out roll made by casting such heat-resistant cast steel is substantially high enough, however, the oxidation resistance and the corrosion resistance to the molten tin are not high enough.
The lift-out roll is always brought into contact with the molten tin heated at 500.degree. to 600.degree. C. whereby the surface of the roll is corroded and oxidized in air to form a rough surface. Accordingly, the uneven pattern of the roll surface called "roll imprint" is transferred onto the surface of the glass ribbon to cause a defect by the formation of uneven or rough surface. This causes a deterioration of quality of the product and an increase of defective products.
When the lift-out roll is exchanged many times, the productivity is decreased so as to cause a cost increase.
The lift-out roll usually means first one to three conveyor rolls disposed at the outlet of the tank for the molten metal. The above-mentioned disadvantages are sometimes found for the conveyor roll or the lehr convey roll which follows the lift-out roll.